1. Field of the Invention
The present invention is related to a thermo mechanical treatment for providing super-plasticity to Al--Li alloy, and more particularly to a thermo mechanical treatment for providing super-plasticity to Al--Li alloy wherein Al--Li alloy is only subjected to a homogenization followed by a warm rolling without a solution treatment and an aging treatment.
2. Description of the Prior Art
As a thermo mechanical treatment method for providing super-plasticity to Al--Li alloy, a kind of light and high strength Al alloy that has become the center of public interest recently, there have been known four kinds of processes representatively, that is, an extruding process, RI-process, Sumitomo-1 and Sumitomo-2 processes. The above-mentioned processes will be described respectively hereinafter.
First, the extruding process is a method for providing super-plasticity to Al--Li alloy by controlling state of precipitation with response to compositions of alloys wherein an ingot is subjected to homogenization and then immediately extruded without intermediate steps. In the process, extrusion speed and extrusion temperature function as factors of super-plasticity.
While the extruding process has advantage in that the thermo mechanical treatment is simple, the process has disadvantage in that the resulting alloy shows a low plastic strain ratio of about 300%.
Secondly, the RI-process is a thermo mechanical treatment developed by Rockwell International Co., U.S.A. The RI-process has been originally developed with the intention of achieving fine grains of 7000 series Al alloy. A study which intended initially to apply the RI-process to Al--Li alloy is disclosed in "Al--Li Alloys II, Met. Soc. AIME, 111-135 (1983) by J. Wordsworth et al., wherein Al--3Cu--2Li--1Mg--0.15Zr alloy manufactured by Lockheed Co. is subjected to double stepped homogenization (first step: for 18 hours at a temperature of 460.degree. C., second step: for 16 hours at a temperature of 500.degree. C.) and then subjected to the RI-process, thereby achieving excellent plastic strain ratio of about 878%.
The heat treatment of the above RI-process comprises a series of steps, that is, a solution treatment, an overaging, a warm rolling and a recrystallization.
However, since the RI-process has been originally developed with the intention of providing super-plasticity to static recrystallization type of Al alloy, it is not suitable to dynamic recrystallization type of Al--Li alloy in super-plasticity and is complicated.
Thirdly, the Sumitomo-1 process modified from the RI-process by H. Yoshida et al. which is disclosed in "4th Int. Al--Li Conf., De Physique, 48, 289-275 (1987)" is carried out such that hot rolled alloy plate is not subjected to a solution treatment and an aging treatment at two steps separately but subjected to the two treatments at one step simultaneously, that is, the alloy plate is annealed at a temperature of 380.degree.-48.degree. C. and cooled in a furnace to allow second phases to be precipitated, and then cold rolled into a strain ratio of 80-90% and recrystallized to achieve fine crystal grains, thereby providing super-plasticity to the alloy plate.
However, although the Sumitomo-1 process has been also developed with the intention of achieving fine crystal grains of 7000 series A1 alloy, it is difficult to provide desirable super-plasticity to Al--Li alloy.
Finally, the Sumitomo-2 process which is modified from the Sumitomo-1 process by H. Yoshida et al. is disclosed in"J. Jpn. Inst. Light Met., V. 39, 817-823 (1989)". In the process, dynamic recrystallization type of Al--Li alloy is subjected to a homogenization and then a controlled rolling to achieve fine crystal grains at the time of a high temperature tension. That is, a 8090 Al--Li alloy ingot is homogenized at a temperature of 520.degree. C. for 24 hours and then rolled under optimal condition of a rolling speed of 3-30 m/min, a temperature of 300.degree. C. and a reduction ratio of 5-15%. The Sumitomo-2 process has advantage in that its thermo mechanical treatment is simple and its super-plasticity is superior to prior art.
However, super-plasticity achieved by the Sumitomo-2 process is considerably reduced under condition beyond the optimal rolling condition. Also, since the rolling condition of the Sumitomo-2 process is not suitable to other Al--Li alloys except for the 8090 Al--Li alloy, its application is limited in a small range.
The reason why the above problems occur in the Sumitomo-2 process is that state of precipitations affecting growth of sub-grains being of importance in super-plastic alloy made by the dynamic recrystallization method varies depending upon the composition of alloy.
For example, in order to achieve excellent super-plasticity, it is preferable to precipitate T.sub.1 (Al.sub.2 CuLi) phases effective in growth and maintenance of fine sub-grains in case of 2090 Al--Li alloy while it is preferable to precipitate T.sub.2 (Al.sub.6 Li.sub.3 Cu) phases in case of 8090 Al--Li alloy.
There has been published results of research showing that when the 2090 and 8090 Al--Li alloy ingots were homogenized and then extruded at various extrusion temperatures and speeds based on the Sumitomo-2 process, the 2090 Al--Li alloy showed an optimal condition of 400.degree. C. and 0.75/min and the 8090 Al--Li alloy showed an optimal condition of 300.degree. C. and 1.5/min. Accordingly, it was found that optimal condition in the Sumitomo-2 process varies depending upon the compositions of Al--Li alloy from the results of research.